US flu activity on a slow decline

Apr 13, 2012 (CIDRAP News) – US flu activity is still hanging on, though some indicators show signs of declining, the US Centers for Disease Control and Prevention (CDC) said today.

Canada also reported ongoing flu activity in some parts of the country, but in Europe influenza viruses appear to be making a more steady retreat.

Parts of the United States are still seeing sporadic activity, the CDC said, including outbreaks nursing homes and other institutions. The agency said the season began late and has been generally mild, but it predicted that flu activity would continue for weeks to come.

Nationally, the percentage of doctor's visits for flulike illness dropped slightly from the previous week, from 1.7% to 1.5%. However, the CDC said the level for one region rose above its baseline, an area of the country that includes Alaska, Idaho, Oregon, and Washington.

Two other markers rose slightly: the percentage of respiratory samples that tested positive for influenza (from 20.5% to 21%) and the percentage of deaths from pneumonia and influenza, which rose from 7.2% to 7.5%, pushing it above the seasonal baseline but keeping it below the epidemic threshold.

The CDC is seeing a fairly wide regional variation in percentage of specimens testing positive for flu, as well as differences in circulating viruses from region to region, and even state to state.

No pediatric flu deaths were reported to the CDC last week, which kept the season's total at 13. The number of states reporting widespread geographic flu spread remained at 10.

So far tested viruses show overall susceptibility to neuraminidase inhibitors, but five more oseltamivir-resistant 2009 H1N1 samples were reported last week, raising the total so far this flu season to seven.

The CDC's flu report included the variant H3N2 (H3N2v) infection in a child reported by Utah officials earlier this week. The CDC released additional details about the case yesterday, and the Canadian Press (CP) reported that the child is a girl who had toured a swine production facility.

The girl's illness is the first H3N2v infection to be reported this year and the 13th case since 2011 involving the same virus, a triple-recombinant swine-origin strain that contains the M gene of the 2009 H1N1 virus.

Elsewhere in the Northern Hemisphere, flu activity appears to have peaked and is declining, the World Health Organization (WHO) said today. Exceptions are some parts of the United States and Canada, it said. Influenza B detections are increasing in North America and in Canadian outpatients started outpacing influenza A in the middle of March.

The WHO said the latter part of the Northern Hemisphere's flu season has been marked by increasing genetic and antigenic diversity of H3N2 viruses.

In Europe flu activity is showing a steady retreat, the European Centre for Disease Prevention and Control (ECDC) said today. No countries reported increasing flu trends, and 18 countries have reported decreasing activity for two consecutive weeks, a sign that their influenza seasons have peaked.

The latest results of respiratory specimen testing showed that the percentage of positive flu samples that are influenza B is 34.8%, the highest proportion so far this season, reflecting an expected pattern at this stage of the flu season.

In a separate report, the ECDC commented on a Eurosurveillance report yesterday that found about a 10% decrease in seasonal flu vaccine effectiveness in Europe compared with last season's. It said the evidence for lower effectiveness found in the study's preliminary findings appears to be credible, given observations from other Northern Hemisphere countries, such as the United States and Spain, along with an apparent excess mortality in older people in some European countries this season.

The drop in vaccine effectiveness could be a result of the late state to the flu season, with immunity to early autumn vaccination that waned over time, but the pattern more likely resulted from the emergence of antigenically drifted H3N2 variants, according to the ECDC.

ECDC director Dr Marc Sprenger said in a statement, "With vaccination remaining the single most effective protective measure against the flu, it is crucial for researchers and vaccine manufacturers to develop more effective vaccines."